Electrolytic preparation of semiconductor compounds



United States Patent 3,419,484 ELECTROLYTIC PREPARATION OF SEMICONDUCTOR COMPOUNDS Frank E. Ammerman, Ann Arbor, and Donald J.

Schindehette, Detroit, Mich., assignors to Chrysler corporation, Highland Park, Mich., a corporation of Delaware N 0 Drawing. Continuation-impart of application Ser. No. 292,487, July 2, 1963. This application Mar. 23, 1966, Ser. No. 536,624

8 Claims. (Cl. 204-86) ABSTRACT OF THE DISCLOSURE An electrodeposition process for directly preparing antimonide, arsenide, telluride semiconductor compounds. The process comprises placing into simultaneous solution as cations the two or more elements which comprise the compound to be synthesized. One of these elements is one of the semi-metals of tellurium, arsenic or antimony and the other element any substance which has a deposition potential less noble than that of the semi-metal when both elements are in solution. The solu tion is then supplied with electrons with the result that the desired compound is formed and deposited on the electron source.

This application is a continuation-in-part of our copending application Ser. No. 292,487, filed July 2, 1963 and now abandoned.

This invention relates to chemical compounds and to the electrosynthesis thereof. More particularly, it is directed to a process whereby certain compounds can be deposited by electrolysis from solutions of the component ions. The process of this invention also makes possible the production of new compounds never heretofore made. i

The electrochemical production of compounds as now known in the art does not rely on the deposition of the compounds as such but depends upon secondary reactions, electrode solution and electrode catalysis as exemplified by:

(1) The production of hypochlorites and chlorates by the production of chlorine in alkaline halides.

(2) The production of permanganates by oxidation using oxygen liberated as an anode.

(3) The solution of lead anodes in the manufacture of white lead.

(4) The solution of cadmium anodes to make cadimum yellow by secondary precipitation.

(5) Internal electrolytic reactions such as produce ethylene glycol and which are indirect electrochemical oxidations in which electrical energy is produced rather than being the means of reaction.

(6) Electrodepositing a layer of metal and then reacting a further element with the metal.

(7) Causing deposit of layers of constitutent elements by vacuum or electrodeposition and reacting the layers.

(8) Reacting a metal surface with a strong acid bath to cause ions thereof to immediately react with those of other elements in the bath to cause precipitation of a compound upon the metal surface for the purpose of coloring or coating the metal such as is done in phosphate coating.

(9) Simultaneously depositing copper and zinc by electrodeposition out of a chemical solution in which there is asserted to be a common deposition potential of these elements to produce a brass alloy plate.

The foregoing are typical of the art in which the object of using electrical energy has been to create solubility or provide a material for reaction or to produce alloy plates predicated upon the simple codeposition of unreacted elements and with no intent of forming compounds except by additional operations. Co-deposition as in brass alloy plating is ascribed by the technical literature and texts as a thermodynamic consideration (the Nernst equation) in which two or more position ions may be simultaneously deposited as individual elements at a cathode whenever the deposition potentials are equivalent and in a proportion dependent upon their concentration in a solution as it affects such potentials. This theory emphasizes the basic need for a common deposition potential and contemplates deposits which will vary in alloy compositions.

These prior processes at best have been difficult to control. The steps performed have been critical. There is no assurance of repetition in result or of even predicting the exact nature of the deposit or to continuously effect a deposit unhampered by uncontrollable factors. Because of these difficulties and imponderables and the cogent desire to operate within the framework of the Nernst equation (as it relates to co-deposition) many previously proposed procedures and hypotheses were abandoned and even those procedures reaching commercial stages while yet employing the basic theory have been complicated and unsure.

Accordingly, it is an object of this invention to provide a process for the electrodeposition of chemical compounds.

A further object is to provide a process whereby compounds are directly produced by the electrolysis of a solution containing the component compound ions.

More specifically, it is an object to provide a process for the production of chemical compounds by electros'ynthesis wherein ions of the component elements of the desired compounds are reduced either by electrolysis of a solution using electrodes and an external current source or by means of immersion or contact deposition wherein electrons are generated within the solution.

Another object is to provide a process for producing semiconductor compounds such as the antimonides, arsenides, and tellurides of specific metallic elements.

Still another object is to provide new and novel chemical compounds not heretofore produced by any known procedure or only by metallurgical procedures.

Other objects, features and advantages of this invention will be apparent from the following description and examples.

The process of the present invention proceeds upon an entirely new and different approach not having the inherent problems of prior theories. It permits of a simple and economical procedure never previously attempted because the procedure is based on principles which are counter to basic chemical theories such as expounded by those who put forth the Nernst equation as the controlling factor in codeposition.

This invention permits the synthesis of certain arsenide, telluride and antimonide compounds of exceptionally high purity by means of a process which is easy and inexpensive to carry out. The process of this invention comprises forming a solution or bath containing cations of the component elements of the compound that it is desired to produce. The solution is then supplied with electrons with the result that the desired compound is formed and deposited on the immediate electron source. In the process of this invention the electrons can be supplied from an external source as by passing current between electrodes, or the electrons can be generated in the solution as by immersion deposition or contact deposition. Both imrnersion and contact deposition or plating are art recognized processes and, hence, will not be elaborated on other than to say that in immersion deposition an element which is less noble than hydrogen, such as aluminum or magnesium, is placed in the solution where it is ionized thereby supplying electrons to the solution. In contact deposition, an inert conductor such as platinum, is brought into direct contact, or connected by means of a wire, with an electron giving material as used in immersion deposition. If an external current is passed between electrodes, the desired compound will be formed and deposited on the cathode. If immersion deposition is used to generate current, the desired compound will deposit on the electron giving active material. Similarly, in contact deposition, the expected compound will be deposited on the inert conductor surface as well as on the active element.

It should be understood that the means by which electrons are supplied to the cation containing solution is unimportant. However, for practical considerations it is preferred to supply the electrons by passing an external current between electrodes immersed in the solution. A further advantage of this method is that all the desired compound is formed on the cathode and is not contaminated by cont-act with the active metal as may be the situation in contact deposition. Accordingly, the balance of the discussion of this invention will be predicated on supplying electrons to the solution by passing an external current between electrodes immersed in the solution.

The compounds which can be formed by the process of this invention are those composed of at least one of the four elements, arsenic, tellurium and antimony (frequently referred to as semi-metals) and a metal whose deposition potential when in the solution containing the semi-metal cation is less noble than that of the semimetal. Stated another way, it is necessary upon electrolysis of the cation containing solution that the semi-metal be deposited upon the cathode prior to deposition of the metal.

The conditions under which the above process is carried out are not critical. For example, successful compound electrodeposition has taken place from baths which were at temperatures ranging from 5 C. to the boiling point of the bath. Likewise, externally generated currents of from about 5 to 3000 amperes per square foot have been used in conjunction with electrical pressure of 2 to 7 volts. Similarly, the pH of bath is not at all critical, however, the bath is generally acidic since an acid medium is a convenient environment for maintaining metallic cations. Surprisingly, cation concentration in the bath is not critical since it has been found that compounds of constant stoichiometric composition are produced up to the very instant that one of the component ions is exhausted from solution. Despite the lack of critical conditions, it has been found convenient for practical considerations to employ a bath temperature in the range of about to 60 C. and a current density of about 5 to 90 amperes per square foot.

The fact that true stoichiometric compounds can be produced by the process of this invention is quite astounding since alloy or compound. electrosynthesis is immediately associated with codeposition and common deposition potential theories. Thus, it has long been taught that two or more metals can only be codeposited at a cathode if their deposition potentials are essentially the same, i.e., less than 0.2 of a volt apart. Thus, it is conventional in any attempt to produce compounds by electrosynthesis to calculate the deposition potential for each of the compound components by means of the Nernst equation. If the deposition potentials are found to differ, as for example by one or two tenths of a volt, then it is assumed that the compound cannot be produced by electrodeposition. Accordingly, compound electrosynthesis remained a laboratory curiosity rather than a practical commercial production technique since the above heretofore used approach was doomed to failure at the outset because of the fact that elements having common deposition potentials in a common solution are exceedingly scarce. The degree to which research workers believed in the need for common deposition potential is exemplified by the art which is replete with various complex salt solutions which represents attempts to provide a medium in which certain elements tend to exhibit a common deposition potential. However, in complete contrast to the above held theories, this invention is based on our discovery that conditions of equal deposition potential and simultaneous deposition at the cathode are to be avoided in producing compounds by electrodeposition.

The process of this invention depends for its success on the use of an element which exhibits the characteristic of forming anions at the cathode and on the use of a second element which is less noble than that element when both such elements are in a common solution or bath. The first requirement is exhibited by each of the elements of tellurium, arsenic and antimony, and therefore, the process of this invention is useful in producing certain compounds of these semi-metals.

In order to more fully clarity the invention, the following mechanism and sequence of events in the production of lead telluride by the process of this invention is provided. It should be understood that the following explanation is equally true of the other compounds capable of being formed by this process. Transference data show that lead and tellurium in a nitric acid solution are present as Pb++ and Te++++ ions surrounded by equivalent dielectric shells of solvent (water) and are about 0.8 volt apart in deposition potential in the solvent. Therefore, at the onset of current flow the tellurium being more noble deposits preferentially on the cathode. This tellurium is absorbed as a positive cation on the cathode surface and is dehydrated and reduced to the neutral state by the electrons provided by an external energy source. Accordingly, the cathode is initially coated with a mono-molecular layer of tellurium.

Once a tellurium surface is established and the cathode film exhausted of tellurium cations, the electrons passing into the tellurium surface from the source are captured to form valence electrons and each adsorbed tellurium cation now becomes an adsorbed tellurium anion. It is not, however, an anion in solution.

An anion on a cathode may be treated either thermodynamically or quantum mechanically and the result in either case is the same. In the first case, it has been established that through it, electrons can be discharged directly into solution without capacitance effects (first order conduction) and in the second case that an anion will discharge its electrons to a cation if the energy level is lower in the cation (chemical reduction). The rates of these reactions are immeasurably faster than ordinary electrode processes and, even more importantly, they are independent of that property known as deposition potential.

The cathode, having preferentially stripped the contiguous electrolyte layer of tellurium, now deposits lead at an extremely high rate with electrons transferring to the cation through the anion. The combination of a lead atom and a tellurium atom is a neutral state and the anionic character of the cathode surface is terminated whereupon the cathode is polarized and normal preferential deposition of tellurium is resumed. The repetition of the series of events creates the layer of lead telluride crystals that are subsequently harvested from the cathode.

Stated otherwise, the process of this invention involves the following order of necessary occurrences.

(1) Transfer of solvated cations to the cathode with a resulting polarization according to commonly accepted principles.

(2) Neutralization of the more noble ion (tellurium, arsenic, antimony).

(3) Conversion of the cathode surface to a layer of adsorbed anions.

(4) Complete depolarization of the cathode with respect to the less noble ion.

(5) Reduction of the less noble ion at the rate of first order electron conductance (metallic).

(6) Polarization of the cathode as the neutral character of the surface is restored.

7. Building of crystal lattices and macrocrystals of the compound of the two or more participating elements through repetition of the polarizing and depolarizing steps.

Though this invention is independent of any specific mechanism or theory of compound synthesis as described above, the described conditions have been confirmed by the production of actual chemical compounds, some of which we believe to be totally new compounds, all of which have utility in thermoelectric or semiconductor devices.

The following examples are provided to further illustrate this invention and should not be construed as a limitation therof. In each of the following examples the compound produced was identified by X-ray diffraction and its purity determined by chemical and spectrographic analysis. Likewise, at least 10 grams and up to 3000 grams of each of the compounds were produced by the method set forth in each of the following examples,

Example 1 Lead telluride (PbTe) was produced as follows:

An aqueous bath was prepared by dissolving the following materials in 100 grams (gms.) of water:

The potassium hydroxide was first dissolved in the water and the remaining ingredients then added to the caustic solution and dissolved therein in any order with stirring. If desired, the tellurium dioxide can be dissolved in an aqueous potassium hydroxide solution in advance and the lead oxide or an aqueous KOH solution thereof can then be added to it at the time of use. The deposit from a freshly prepared bath will have less residual tellurium (free and uncombined) than a bath which has been stored.

The bath was then subjected to electrolysis while at room temperature (approximately 22 C.) using a lead anode and a steel cathode and employing a current dens ty of 10 to amperes per square foot. A deposit formed from the supernatant liquid at room temperature and the bath may be gently stirred during electrolysis without danger of contaminating the cathode deposit with precipitates that may form at the anode during plating.

At suitable intervals the cathode was removed from the bath and the sponge deposit removed therefrom. A quantity of the sponge was washed, dried, and then compacted at 50,000 pounds per square inch (p.s.i.), X-ray dilfraction and tests to determine Seeburg Coefiicient served to identify this compacted material as substantially pure P-type lead telluride.

It was noted that the addition of One gram of potassium fluoride to the above set forth bath increased the amount of lead telluride produced and made the deposit on the cathode more uniform.

As mentioned earlier, the concentration of component ions in the bath is not critical in the production of the compound. However, care must be taken to insure that cations of both components are in solution. For example, if the above described bath contained only 0.5 gram of tellurium dioxide instead of 5.0, only lead would probably be deposited due to the extremely low solubility of tellurium dioxide. Likewise, the choice of solvent is important and must be taken into account. Thus, sodium hydroxide may be substituted for potassium hydroxide in the above bath but the solubility of the components therein is somewhat less at room temperature.

6 Example 2 Gms. Lead (Pb) (as metal) 3 Tellurium (Te) (as metal) 3 Nitric acid (HNO (conc.) (excess) 5 The source of lead may be either lead nitrate or lead monoxide which is taken up in nitric acid, and the source of the tellurium may be either tellurium dioxide or tellurium metal both of which may be taken up in nitric acid. The quantity of nitric acid is identified as excess since the amount was over and above that needed to dissolve the lead and tellurium. The concentrations outlined above may be reduced to 10% of that stated and still be sufiicient to provide lead and tellurium cations in solution and the ratio of lead to tellurium varied from 1:2 to 5000:1 without affecting the chemical nature of the product. The excess nitric acid serves a double purpose in that (a) it prevents hydrolysis of the nitrate of tellurium, and (b) redissolves any lead telluride that may fall from the cathode during deposition and subsequent handling of the electrode.

Upon electrolysis of this solution when it was at room temperature (about 20 C.) using a current of about 20 amperes per square foot and platinum electrodes (with respect to the cathode, tantalum may be substituted for platinum and lead can be used as an anode) a graphitelike deposit was received at the cathode. Scraped from the cathode and washed and dried, this deposit was found to have a dendritic macrostructure by electron microscopy and the lattice structure of lead telluride by X-ray diffraction analysis. In fact, the lattice parameter proved to be slightly smaller than standard indicating a closer stoichiom-etric ratio than PbTe produced by metallurgical means.

A quantity of lead telluride was prepared from this bathcomposition and after compacting into a suitable slug at 50,000 p.s.i., was tested to determine its Seeburg coefficient and found to be P-type lead telluride. It was possible to melt (zone or total) such a slug and improve its electrical conductivity without apparent chemical change or damage to other properties.

Bath concentrations were maintained during electrolysis by simply adding metallic tellurium and lead nitrate to the bath while operating. The oxides of lead and tellurium could be substituted. As nitric acid was depleted by electrolysis to hydrogen and oxygen it was replaced by direct addition.

It is noted that electrolysis of the above solution was found to proceed satisfactorily at temperatures from 10 to 40 degrees centigrade and at currents of 5 to 40 amperes per square foot.

Example 3 Copper telluride (CuTe) was produced as follows:

An aqueous solution was prepared containing the following ingredients in essentially the proportions indicated dissolved in gms. of water:

Gms. Copper cyanide (CuCN) 2 Sodium cyanide (NaCN) 3 Caustic soda (NaOH) 3 Tellurium dioxide (TeO 5 The tellurium dioxide was first dissolved in a portion of the sodium hydroxide and water and then combined with the balance of the solution which contained the copper cyanide and sodium cyanide in water.

This solution was then electrolysed at room temperature using a copper anode and steel cathode and a current density of about 10 amperes per square foot. A material was deposited at the cathode and analysis disclosed it to be copper telluride with residual traces of tellurium. Copper telluride was also produced from the above bath using a steel anode and cathode, and inert anodes such as platinum.

Example 4 Mercuric telluride (HgTe) was produced as follows:

An aqueous solution was prepared by dissolving in 100 grns. of water essentially the following amounts of ingredients:

Mercuric chloride (HgCI 5 grns. Tellurium (as metal) 5 grns. Hydrochloric acid (excess) l0 mls. of commercial concentrated acid (37 38% HCl The solution was prepared by first dissolving the tellurium metal in concentrated hydrochloric acid and a portion of the water. The mercuric chloride was dissolved in an aliquot of the water and the solutions were then combined and adjusted to the stated proportions. The excess acid was that over and above the amount needed to dissolve the metal.

Inert carbon electrodes were inserted in the bath which was at room temperature and a current of approximately amperes per square foot was passed, whereupon mercuric telluride was received at the cathode at room temperature.

Subsequent analysis of the deposit disclosed it to be pure mercuric telluride free of residual elements. The initial deposit from a freshly prepared bath may contain some free mercury but this condition soon disappears. Mercuric telluride deposits were also obtained from the above bath when electrodes fabricated from stainless steel and platinum were used.

Example 5 Iron telluride known as FeTe was produced from the following aqueous solution by observing certain described procedures.

An all chloride iron bath was prepared by dissolving 45 grns. of ferrous chloride (FeCI and gms. of calcium chloride in 100 gms. of Water. This bath was electrolysed at about 88 C. using an iron anode and steel cathode until the solution was pale green (completely ferrous). At this point commercial grade metallic tellurium was dissolved in concentrated hydrochloric acid and added to the bath in such a manner as to cause a concentration of 5 gms. of tellurium per 100 gms. of water.

Upon the electrolysis of the resulting solution at 88 C. using a current of 40 amperes per square foot, a deposit formed upon the cathode which was subsequently analyzed and found to be iron telluride FeTe or which may be written FeTe.Te, i.e., tellurium forming intermetallie compound with FeTe (ferrous telluride). Metallic iron was found to be present as a residual deposit.

The prolonged electroylsis of the iron bath was used to increase the efliciency of the bath to prevent gas bubbles from dislodging the deposited compound.

Example 6 Nickel telluride (NiTe was produced by modifying a Watts nickel bath as follows:

To a standard Watts nickel solution of the following concentrations of salts and acid per 100 grns. of water:

Gms. Nickel sulfate (NiSO 21 Nickel chloride (NiCl 5 Boric acid (H BO 3 was added tellurium prepared by dissolving said element in sulfuric acid so that the tellurium was present with the above ingredients in the amount of 3 grns. per gms. of water. The pH of the above mixture was reduced to less than 1.5 to keep the tellurium in solution.

When the solution was electrolysed by means of a nickel anode and brass cathode, a deposit was formed that was found to be nickel telluride and a residue of tellurium. Electrolysis was carried out at about 49 C. and 30 amperes per square foot. Wide variation in tellurium concentration was not found to affect the nature of the deposit.

This telluride also approximates a phase variant which could be described as NiTe.Te as in the system of iron.

Example 7 Tin telluride (SnTe) was readily produced from an alkaline stannate solution containing the following quantities of ingredients per 100 gms. of water:

Gms. Pottasium stannate (K SnO 10 Potassium hydroxide (KOH) 3 Tellurium dioxide (TeO 5 The tellurium dioxide was stirred into the solution of the potassium hydroxide and the potassium stannate then added.

Upon electrolysis with steel electrodes at 20 amperes per square foot and at a temperature of about 88 C. the deposit formed at the cathode had the composition SnTe (tin telluride) plus some residual tin.

Example 8 Bismuth telluride (Bi Te was produced from an acid chloride solution created by dissolving bismuth trioxide (Bi O and metallic tellurium in hydrochloric acid and then adjusting the resultant mixture with water and hydrochloric acid to the following composition (expressed as the grains present per 100 grns. of water).

Gms. Bismuth (as metal) -1 3 Tellurium (as metal) 3 Hydrochloric acid (37%) (excess) 5 Using platinum electrodes, bismuth telluride (Bi Te was obtained as a deposit at the cathode upon passage of current at 10 amperes per square foot through the above bath which was at room temperature. Some free bismuth was obtained upon first operating a newly prepared solution but afterwards bismuth telluride was the only product.

Example 9 Silver telluride of the naturally occurring form Ag Te (Empressite), which is predicated in the literature to be a phase of Ag Te and AgTe, was produced from a solution containing the following concentration of constituents per 100 gms. of water:

Gms. Silver cyanide (AgCN) 2 Potassium cyanide (KCN) 5 Potassium carbonate (K CO 4 Caustic potash (KOH) 6 Tellurium dioxide (TeO 4 All ingredients were dissolved together with the potassium cyanide being necessary for the solution of silver cyanide and the caustic potash being necessary for the solution of the tellurium dioxide.

Using electrodes of steel and passing a current of 10 to 15 amperes per square foot, silver telluride was deposited on the cathode. This operation was carried on at room temperature without agitation of the cathode or solution. Diffraction analysis of the deposit showed it to be identical with the naturally occurring mineral, Empressite. Silver telluride was also obtained from this bath using platinum electrodes.

9 Example 10 Example 11 Thallium telluride (TlTe) was produced as follows:

In 100 gms. of water there was dissolved 3 gms. of thallous sulfate (Tl CO Subsequently 3 gms. of tellurium was added to 10 gms. of concentrated sulfuric acid (H 50 which solution was added to the aqueous solution of thallous sulfate previously prepared.

By means of inert electrodes (platinum) current was passed though the resulting solution at the rate of 5 to 15 amperes per square foot, From the freshly prepared solution some metallic thallium was produced at the cathode but after standing 24 hours, the action of electrolysis of the bath produced, as determined by X-ray diffraction, only thallium telluride (TlTe) in large crystalline graphitic plates.

These crystals were removed from the cathode at inter vals, washed, dried and compacted at 50,000 p.s.i. and were found through determination of the Seeburg coefficient, to have the properties of a P-type thermoelectric semiconductor.

Example 12 Copper arsenide (Cu As, 18 form) was produced by the following method.

A conventional cyanide copper bath was prepared by dissolving in 100 grams of water in the given order the following ingredients.

Gms. Caustic soda (NaOH) 1 Sodium cyanide (NaCN) 3 Copper cyanide (CuCN) 2 and a conventional arsenic bath was prepared by dissolving in 100 grams of water in the given order:

Gms. Caustic soda (NaOH) M 6 Arsenic trioxide (As The arsenide of tin (Sn As was produced as follows: In 100 gms. of water was dissolved the following salts:

Gms.

Potassium stannate (K SnO 10 Caustic potash (KOH) 10 To this solution was added:

Arsenic trioxide (As O 5 The resulting solution was electrolysed with steel electrodes and the cathode deposit was found to be tin arsenide (Sn As The electrolysis was carried on at room temperature and at 10 amperes per square foot. A trace of arsenic was noted in the deposit accompanying the arsenide. The same result was achieved using platinum electrodes.

Example 14 The compound indium arsenide (InAs) was produced as follows:

The following ingredients were dissolved in 100 gms. of water in the following order:

Gms. Indium chloride (InCl 6 Potassium cyanide (KCN) 28 Caustic potash (KOH) 4 Separately, and in gms. of water, were dissolved the following:

Gms. Arsenic trioxide (As O 5 Caustic potash (KOH) 5 These two solutions were combined and allowed to stabilize for 24 hours.

The solution was electrolysed at room temperature using 15 amperes per square foot and steel electrodes and a coherent brittle deposit was formed at the cathode. At eight hour intervals this coherent film was peeled from the cathode, washed, dried and ground into powder. The film was analysed and found to be indium. arsenide (InAs) plus a trace of indium.

A quantity of the powdered plate was compacted at 50,000 p.s.i. and found to have the properties of a thermoelectric semiconductor compound (p-type).

It was observed that this bath could be operated to the exhaustion of the indium. As the indium concentration fell the rate of product deposition also fell, but indium arsenide was still formed.

Example 15 Silver antimonide of the structure Ag Sb was prepared according to the following procedure:

A conventional silver cyanide bath with an excess of caustic was prepared by dissolving in 100 gms. of water the following ingredients in order:

Gms.

Potassium carbontae (K CO 4 Potassium cyanide (KCN) 5 Silver cyanide (AgCN) 2 Caustic potash (KOH) 6 To this was added:

Antimony trioxide (Sb O 3 The resulting solution was electrolysed at room temperature using steel electrodes and a current of 15 amperes per square foot.

The metallic deposit on the cathode was removed and found to be silver antimonide (Ag sb) plus an alloy of antimony in silver as a residue accompanying the antimonide. Silver antimonide was produced by this same procedure using platinum electrodes in place of steel.

Example 16 A ternary compound of lead-tellurium-selenium idcntified as Pb TeSe was produced by the following steps.

To a solution of the nitrates of lead and tellurium as described in Example 2, there was added 1 gm. of selenium dioxide (SeO Electrolysis was effected by passing a current of 10-15 amperes per square foot through platinum electrodes at room temperature. The cathode deposit was found to be a coating of crystals intermediate in appearance between that of lead telluride and that of lead selenide. Electron micrographs at 10,000X showed the m'acrostructure to be intermediate between lead telluride and lead selenide deposits. X-ray diffraction analysis revealed lattice spacings corresponding to the proportions Pb TeSe.

A quantity of this ternary was prepared, compacted at 50,000 p.s.i. and found to have thermoelectric semiconductor properties (p-type).

served that a constant deposit composition of Pb TeSe was obtained from the bath cited in Example 16 even when the selenium content was substantially reduced and that Pb TeSe would be formed preferentially to any other combination of ions until the selenium was exhausted:

To a solution composed of 100 gms. of Water, 2 gms. of lead nitrate (Pb(NO and 2 gms. of selenium dioxide (SeO was added small increments of tellurium dissolved in nitric acid of the order of 0.1 gm. of metallic tellurium per 100 gms. of water. The nitric acid was used in amount just sufficient to dissolve all of the metallic tellurium. After the electrolysis of this thus prepared solution by normal means, it was found that a series of alloys of tellurium in lead selenide were formed including the compound state represented by Pb TeSe. Thus, the conditions obtaining in the bath are found to exclude alloys of selenium in lead telluride but permit alloys of tellurium in lead selenide.

Example 18 The alloy of a metal in lead telluride was produced by incorporating zinc in the crystal lattice of PbTe through the following procedure.

To a nit-rate bath of lead and tellerium, prepared as previously described in Example 2, was added zinc nitrate (derived from zinc oxide) at the concentration of 2 gms. per 100 gms. of water.

Upon electrolysis with platinum electrodes at a current density -l5 amperes per square foot at room temperature, a deposit was obtained which showed changes in lattice spacing indicative of the incorporation of zinc atoms with the crystal structure.

A quantity of the alloy was prepared and compacted at 50,000 lbs. per square inch and then tested electronically. The test showed the semiconductor parameters of the alloy were markedly altered from that of lead telluride.

Example 19 To incorporate a halide, specifically ionide, within the lattice of lead telluride the following procedure was followed:

To 100 ml. of 10% nitric acid in water was added an excess of crystals of iodine such that the solution would become saturated with iodine. To this solution was added 0.5 gm. of lead nitrate and the whole was allowed to stand 24 hours. This solution was then decanted into 500 ml. of lead-tellurium nitrate solution prepared as previously described in Example 2.

Electrolysis of this solution at room temperature using platinum electrodes and a current of l015 amperes per square foot initially produced sites at which lead iodide was being deposited. This ceased on continued electrolysis and only a physically modified lead telluride deposit appeared at the cathode. X-ray diffraction and spectrographic analysis of the deposit was unable to detect the presence of iodine because of the similarity of iodine and tellerium. However, the crystal was that of lead telluride and chemical analysis showed substantial amounts of iodine present indicating the presence of the iodine occupying tellerium sites or the condition of lead iodide dissolved in lead telluride as well as direct deposition of lead iodide under the conditions heretofore described.

Example 20 In order to convert the graphite-like crystals of lead telluride into a coherent metallic plate a very small quantity of bismuth trioxide (Bi O was added to the basic lead-tellerium nitrate solution (Example 2). The amount employed was 0.1 gm. of Mi O per 100 ml. of solution. The deposit at 15 amperes per square foot was a very bright, hard coherent metallic plate which was found to be lead telluride. An addition of beryllium oxide (BeO) in the same quantity and under the same conditions of deposition also caused the previously loose crystalline 12. deposit of lead telluride to become coherent and platelike.

Example 21 Antimony thallide (SbgTlq) was produced from a bath containing 3 grams per liter of thallous sulfate, 1 gram per liter of free or excess concentrated sulfuric acid and antimony sulfate in an amount sufficient to form a saturated solution. Upon electrolysis of this bath using platinum electrodes, a bath temperature of about 20 C. and a current in the range of 15 to 20 amperes per square foot, a deposit formed upon the cathode which, upon analysis, was found to be antimony thallide.

Example 22 Cadmium arsenide (Cd As was produced as follows: An aqueous solution was prepared by dissolving in water essentially the following amounts of ingredients:

Oz./gal. Cadmium oxide 1.5 Sodium cyanide 8.0 Arsenic trioxide 8.0 Caustic soda 8.0

This solution was electrolysed at room temperature using platinum electrodes and a current of about 15 amperes per square foot. A coherent deposit of cadmium arsenide was formed at the cathode.

Example 23 Nickel arsenide having the formula fiNi As was deposited from a bath made up of about 30 grams per liter of inckel chloride; 20 milliliters per liter of excess concentrated hydrochloric acid and arsenic trioxide in an amount suflicient to form a saturated solution. The deposit was produced by electrolysis of the above solution while at room temperature using platinum electrodes and a current of about 17 amperes per square foot.

Example 24 Palladium telluride (PdTe) was electrodeposited from a bath made up of about 10 grams per liter of palladium chloride, 10 milliliters per liter of excess concentrated hydrochloric acid and commercial grade tellurium metal which had been dissolved in hydrochloric acid so that its concentration in the final bath solution was about 10 grams per liter. This solution was electrolysed at room temperature by means of platinum electrodes using a current of 20-25 amperes per square foot.

Example 25 The following bath was electrolysed while at room temperature using platinum electrodes and a current of about 3035 amperes per square foot:

Oz./gal. Nickel sulfate 12 Antimony chloride 15 Hydrochloric acid 3 A deposit of nickel antimonide (NiSb was formed on the cathode.

Example 26 Rhenium ditelluride (ReTe was deposited by the electrolysis of a bath made up of 5 grams per liter of perrhenic acid, 5 grams per liter of potassium sulfate, 3.5 grams per liter of excess concentrated sulfuric acid and sufiicient tellurium dioxide to produce a concentration of tellurium metal of 2 grams per liter in the final bath solution. The electrolysis of the bath was carried out at room temperature using platinum electrodes and a current of about 10 to 15 amperes per square foot.

Example 27 Cobalt antimonide (CoSb) was produced by the electrolysis of a bath made up of 12 ounces per gallon of cobalt sulfate, 15 ounces per gallon of antimony chloride and 3 ounces per gallon of hydrochloric acid. Platinum electrodes and a current of about 20 30 amperes per square foot were used to carry out the electrolysis.

Example 28 Example 29 Palladium antimonide (PbSb was produced by the electrolysis of a hath made up by dissolving palladium chloride and antimony trioxide in concentrated hydrochloric acid (37%) and then adjusting the resultant mixture with water and hydrochloric acid to the following composition:

Antimony (as metal) grams/liter 10 Palladium (as metal) do 10 Hydrochloric acid (excess) milliters 10 The electroylsis was carried out at room temperature using platinum electrodes and a current of about 20-25 amperes per square foot.

Example 30 Tin antimonide (SnSb) was electrodeposited from a solution made up of:

Oz./gal. Potassium stannate 10 Potassium hydroxide Antimony hydroxide 2 The above solution was heated to a temperature of about 90 C. and electrolysed using platinum electrodes and a current of about -15 amperes per square foot.

Example 31 Gallium arsenide (GaAs) was produced by electrolysis of a bath which was prepared by adding gallium chloride to a solution of Oz./ gal. Arsenic trioxide 16 Sodium hydroxide 16 Sodium cyanide 1 The gallium chloride was added until it began to precipate from the above solution and the resulting solution was then electrolysed at room temperature using a platinum anode, a steel cathode and a current of about 20-25 amperes per square foot.

The following examples illustrate the production of certain chemical compounds by the process of this invention wherein reduction occurs through electrons generated within the cation containing solution. Any of the electron giving or active metals as are well known in the immersion and contact plating art can be used in this invention. The preferred active metals, however, are those which are less noble than hydrogen when in common solution with the compound components. Thus, the active metal used in this invention is sharply distinguished from the compound components which are more noble than hydrogen.

Example 32 Silver telluride of the form Ag Te (hessite) was produced from a solution containing the following concentration of compounds per 100 grams of water:

Grams Silver cyanide (AgCN) 2 Potassium cyanide (KCN) 5 Potassium carbonate (K CO 4 Caustic potash (KOH) 6 Tellurium dioxide (TeO 4 Altuninum granules of QR reagent grade were placed in the above solution which was at room temperature. Silver telluride formed on the surface of the aluminum granules and as the aluminum dissolved shells of silver telluride were collected which were then pressed into a slug weighing 10.3 grams. Analysis of this material showed it to be a P-type semiconductor of silver telluride (Ag Te) Example 33 Lead telluride was produced from a solution prepared by dissolving 15.0 grams of potassium hydroxide, 7.5 grams of lead oxide and 5.0 grams of tellurium dioxide in grams of water. A strip of alumiun foil was placed in the solution and lead telluride was observed to be deposited thereon while the solution was at room temperature.

Example 34 Lead telluride was also produced as an immersion plate on metallic lead by immersing clean lead sheet in a room temperature solution prepared by dissolving 3 grams of lead as lead nitrate, 3 grams of tellurium and 5 grams of excess concentrated nitric acid in 100 grams of water.

Example 35 The compound bismuth telluride (Bi Te was prepared by immersing granular aluminum in a room temperature acid chloride solution prepared by dissolving bismuth trioxide and metallic tellurium in hydrochloric acid and adjusting the resultant mixture with water and hydrochloric acid to the following composition expressed as the grams present per 100 grams of water:

Grams Bismuth (as metal) 3 Tellurium (as metal) 3 Hydrochloric acid (37%) (excess) 5 Bismuth telluride was also prepared from the above solution by immersing a clean metallic bismuth sheet therein. In both instances, the compound deposited on the active metal was compacted and analysed as P-type bismuth telluride.

Example 36 Mercuric telluride (HgTe) was produced from a solution prepared, as set forth above in Example 4, by dissolving in 100 grams of water essentially the following amounts of indgredients:

Mercuric chloride grams 5 Tellurium (as metal) do 5 Concentrated hydrochloric acid (37% HCl) (excess) mils 10 Aluminum foil was then immersed in the above acid bath which was at room temperature and a compound was deposited thereon. Analysis of this compound showed it to be P-type mercuric telluride.

Example 37 Thallium telluride (TlTc) was produced by dissolving 3 grams of thallous sulfate (Tl SO in 100 grams of water and then adding to this aqueous solution a solution made up by dissolving 3 grams of tellurium in 10 grams of concentrated sulfuric acid.

Granules of aluminum which had been rinsed in hydrochloric acid were then suspended in the above room temperature solution and the compound deposited thereon was subsequently identified as thallous sulfate. The aluminum granules were rinsed in hydrochloric acid to remove the oxide therefrom since sulfuric acid is not effective for this purpose.

Example 38 In order to ascertain if the contact plating technique was applicable in the process of this invention, each of the above experiments as set forth in Examples 32-37 were repeated except that in each case an inert platinum conductor was attached by means of a wire to the active metal. This couple was then immersed in the solutions which were at room temperature. In each instance, the desired compound was deposited on the platinum surface as well as on the active element.

From the above discussion of this invention and the illustrative examples, it will be apparent that the objects set forth hereinabove have been met. This invention provides an extremely easy and economical method of producing certain semiconductor compounds by the electrolysis of aqueous solutions. Moreover, this process of this invention does not require the use of any given voltage, pH, current, bath temperature or ion concentration. It does, however, require the simultaneous solution as cations of the two or more elements which compose the compound which is to be synthesized. One of these elements at least must be a semi-metal selected from tellurium, arsenic or antimony and the other element can be any substance which exists as a cation in the mutual solution and which has a deposition potential less noble than that of the semi-metal when both elements are in solution as cations. An appreciation of the scope of this invention can be gained from the following groupings of semi-metal and metal elements which have been used to produce compounds. Thus, excellent results have been achieved using the process of this invention in making compounds composed of arsenic and one of the metals of the group cadmium, copper, indium, nickel, tin and gallium. Similarly, the semi-metal tellurium has been compounded with one of the metals from the group bismuth, copper, iron, lead, mercury, palladium, rhenium, silver, thallium and tin, while the semi-metal antimony has been combined with one of the metals selected from the group of cobalt, copper, nickel, palladium, Silver, thallium and tin.

Accordingly, unlike prior teachings, the process of this invention requires a divergence or difference in the deposition potential of the component compound elements so as to allow initial deposition of the semi-metal and a large enough energy gap to allow its anion formation before deposition of the less noble metal can occur. Preferably, this difference in deposition potential is 0.2 volt or greater.

It will also be apparent from the foregoing description of this invention and the examples given, that various changes, substitutions, and modifications in the specific compositions and processing of the invention will suggest themselves to those skilled in the art without departing from the spirit and intent of the invention. Moreover, it will be evident that many additional compounds not specifically described may be made by the described process. All such changes, modifications, substitutions and additions as may come within the purview of this invention and the following claims are therefore contemplated.

We claim:

1. A process for producing arsenide, telluride, and antimonide compounds in sponge form at a cathode which comprises (1) forming an aqueous solution containing (a) cations of an element selected from the group consisting of arsenic, tellurium, and antimony and (b) cations of a metal whose deposition potential in the solution is less noble than the deposition potential of said element in the solution, (2) immersing an anode and cathode electrode in said solution, (3) passing a current between said electrodes and through said solution and forming a surface on the cathode of said element, (4) continuing to pass said current to cause said surface to become anionic and depolarized with respect to the metal cation, and (5 electrically reducing the metal cation and depositing said metal cation upon the anionic surface of the cathode and thereafter removing said deposit from the cathode.

2. A process according to claim \1 wherein the deposition potential of the metal cation is less noble than the deposition potential of the element cation by at least about 0.2 volt, said deposition potentials being measured in references to the solution containing both of said metal and element cations.

3. A process according to claim 1 wherein the metal cations are of the metals selected from the group consisting of bismuth, cadmium, cobalt, copper, gallium, indium, iron, lead, mercury, nickel, palladium, rhenium, silver, thallium and tin.

4. A process according to claim 1 wherein the solution contains (a) cations of arsenic and (b) cations of a metal selected from the group consisting of cadmium, copper, indium, nickel, tin and gallium.

5. A process according to claim 1 wherein the solution contains (a) cations of tellurium and (b) cations of a metal selected from the group consisting of bismuth, copper, iron, lead, mercury, nickel, palladium, rhenium, silver, thallium and tin.

6. A process according to claim 1 wherein the solution contains (a) cations of antimony and (b) cations of a metal selected from the group consisting of cobalt, copper, nickel, palladium, silver, thallium and tin.

7. A process according to claim 1 for producing leadtelluride (PbTe) wherein the solution contains (a) cations of tellurium and (b) cations of lead.

8. A process according to claim 1 for producing indium arsenide (InAs) wherein the solution contains (a) cations of arsenic and (b) cations of indium.

References Cited UNITED STATES PATENTS 2,258,963 10/1941 Woll et al. 204-56 2,811,571 10/1957 Fritts et al 13686 3,023,079 2/1962 Kulifay 2350 3,130,137 4/1964 Araki et al. 2044-3 3,271,276 9/1966 Guilio et al 204-43 OTHER REFERENCES Journal Electrochemical Society, vol. 106, No. 8, pp. 685 to 689, Arrays of Inorganic Semiconducting Compounds, A. J. Cornish, August 1959.

JOHN H. MACK, Primary Examiner.

HOWARD M. FLOURNOY, Assistant Examiner. 

